Hi, I don't know how I did that but I have 2 hidden photos :
when I click on the 'All Files' category, I have the following message in the status bar : '1 of 4180 image(s) (2 hidden)'
I know that it is possible to display it by using ctrl+H, but I don't know how to 'unhide' it and, more important, how did I manage to hide it !